Zeno called his mentor, Geri to discuss last quarter’s results. “Geri,” Zeno complained, “you know I’m a photographer and not an accountant. Each month last quarter I sold 50 portraits, but that accountant you recommended claims my monthly productivity was different each month. How can that be?”
 
Geri replied, “Tell me more about your photography operation and let me see if I can explain.”

“Well, the first month I bought and used ten rolls of film. Each roll of film has the ability to take 20 pictures. I’d take 6 shots of each person and use one roll for three customers, losing those last two shots. The next month, I decided to see if I could just take 5 pictures per person and make better use of my film. I used only nine rolls that month.

“Month three? My accountant claims my productivity went up, but I used only 8 rolls of film and still took 5 shots of each subject. I’m not sure how that could be. All I really did differently was to fool around with the lighting. I found by shifting my lights I could take ten years off your age. That was the first month where I sold every single portrait I did of women “of a certain age” if you know what I mean.”

How can Geri explain this to Zeno? What was Zeno’s monthly productivity?